Advances in integrated circuit technology have enabled discrete components to embody an increasingly high degree of functionality in an increasingly small unit of area. Even so, printed circuit boards remain the method of choice for interconnecting these discrete components. Consequently, designers of printed circuit boards have been forced to decrease trace widths and add conductor layers in order to accommodate the increased demand for off-chip connections and circuit traces per board. The result has been a dramatic rise in printed circuit board density and complexity.
These increases in density and complexity have presented challenges in the field of circuit board testing. For example, the higher the complexity of the printed circuit board, the higher the rate of board failures and the greater the cost of defects occurring at each stage of the assembly. Therefore, testing methods and apparatus have been developed to test not only completed printed circuit board assemblies, but also to test the bare boards themselves before the discrete components are attached.
The most common of these testing apparatus is the so-called "bed of nails" test fixture. A bed of nails test fixture is characterized by a supporting plate that holds numerous nail-like test probes firmly upright. The test fixture positions the probes in relation to the printed circuit board so as to engage a particular circuit node when the bed of nails and the printed circuit board are brought into contact with one another, usually by means of either a vacuum-operated or a pneumatic force. In turn, each of the individual probes is spring loaded. Thus, as the circuit board and test probes engage, the springs in the probes depress slightly. In many typical applications, a compressive force between three and eight ounces is desirable on each probe to ensure connections that are reliable and not highly resistive. Unfortunately, because multiple probes are used simultaneously, such fixtures often apply enormous forces to the printed circuit boards under test. For example, if four-ounces of test probe force were applied to each of the pins on a sixty-four pin microprocessor package, the aggregate force would equal sixteen pounds applied to a very small area of the printed circuit board, and in a direction orthogonal to the plane of the board. These types of forces cause deformation of the printed circuit board. Very frequently, the deformation causes damage.
An alternative method for gaining access to test nodes on printed circuit boards is to build one or more plug-and-socket-type test connectors onto the board for testing purposes. With this done, testing may be accomplished by simply attaching an appropriate cable to the plug-and-socket-type test connector, conducting the test, and then removing the test cable from the plug-and-socket-type connector. This method has disadvantages also. First, it decreases the throughput of the testing operation because plug-and-socket-type connectors usually require that test cables be attached and removed by hand. Second, the cost of the plug-and-socket-type connectors themselves can be prohibitive in a high-volume production. Third, adding plug-and-socket-type connectors to the board represents an additional source of potential defects and a statistically measurable cost for rework and replacement.
It is therefore an object of the present invention to gain reliable electrical access to test nodes on a printed circuit board without applying destructive forces to the circuit board.
It is a further object of the present invention to gain reliable electrical access to test nodes on a printed circuit board without the requirement of building plug-and-socket-type test connectors onto the board.